1. Field of the Invention
The present invention relates to an electric power steering system which assists a steering operation by driving an electric motor according to a steering torque and transmitting a driving force generated by the electric motor to a steering mechanism. The invention further relates to a control system for such an electric power steering system.
2. Description of Related Art
In an electric power steering system for assisting a steering operation by utilizing a driving force generated by an electric motor, the electric motor is driven on the basis of a steering torque applied to a steering wheel, and the driving force generated by the electric motor is transmitted to a steering mechanism.
The electrical construction of a prior art electric power steering system is shown in FIG. 3. The electric power steering system controls an electric motor 58 for applying a steering assist force to a steering mechanism by a controller 50 comprised of a microcomputer. An output signal of a torque sensor 57 for detecting a steering torque applied to a steering wheel is inputted to the controller 50. The analog signal outputted from the torque sensor 57 is converted into a digital signal by an analog/digital (A/D) convertor 51. The controller 50 determines a control command value corresponding to an electric current to be applied to the electric motor 58 on the basis of the steering torque T in a digital form, and drives the electric motor 58 via a motor driving circuit 55 on the basis of the control command value.
The controller 50 is adapted to perform every computation therein on a digital basis. That is, the controller 50 performs software-based computations according to predetermined operation programs, thereby functioning as an assist controlling section 52, an inertia compensating section 53 and an adder section 54.
The assist controlling section 52 determines a target current value I according to the steering torque T. The inertia compensating section 53 outputs a compensation current value xcex94I according to a time-based differential value xcex94T of the steering torque T. The compensation current value xcex94I is an electric current value for compensating for a response delay occurring due to the inertia of the steering mechanism and the electric motor 58. The adder section 54 adds the compensation current value xcex94I from the inertia compensating section 53 to the target current value I from the assist controlling section 52 to provide the control command value I+xcex94I. The motor driving circuit 55 is controlled on the basis of the control command value I+xcex94I, whereby the electric motor 58 generates the steering assist force according to the steering torque T with a satisfactory responsiveness.
FIG. 4 is a diagram showing a relationship between the time-based differential valuexcex94T of the steering torque T and the compensation current valuexcex94I. The inertia compensating section 53 outputs the compensation current value xcex94I which is, for example, proportional to the time-based differential valuexcex94T of the steering torque T. However, all the computations to be performed in the controller 50 are program-based digital computations, so that the time-based differential value xcex94T is merely a discrete value on a one-bit basis and, hence, the compensation current valuexcex94I is also a discrete value. Therefore, the compensation current value xcex94I changes stepwise with respect to the time-based differential value xcex94T.
Where the steering torque T changes sufficiently rapidly, the electric motor 58 can properly be controlled even if the compensation current value xcex94I changes stepwise with respect to the time-based differential value xcex94T. Where the steering torque T changes very slowly with time, on the contrary, the target current value I outputted from the assist controlling section 52 changes stepwise by a very small amount in a longer cycle as shown in FIG. 5A. Accordingly, a pulse indicative of the compensation current value xcex94I is generated for a time-based differential value of xcex94T=1 bit in a relatively long cycle, as shown in FIG. 5B, by the inertia compensating section 53. In this case, the control command value I+xcex94I obtained by the addition of the target current value I and the compensation current value xcex94I in the adder section 54 appears as a pulse in a pulsed waveform in a relatively long cycle as shown in FIG. 5C. When a pulsed electric current is applied to the electric motor 58 according to the control command value, resonance occurs between the electric motor 58 and the steering mechanism, resulting in uncomfortable vibrations and noises.
It is therefore an object of the present invention to provide an electric power steering control system which can effectively suppress the vibrations and the noises.
It is another object of the present invention to provide an electric power steering system which can effectively suppress the vibrations and the noises.
The electric power steering control system according to the present invention is a control system for an electric power steering system which is adapted to generate a steering assist force by an electric motor driven on the basis of a steering torque applied to an operation member, and comprises: a target current determining circuit for generating a target current value according to the steering torque; a compensation value determining circuit for generating a compensation current value according to a time-based differential value of the steering torque; a waveform transforming circuit for performing a waveform transformation to convert the compensation current value generated by the compensation value determining circuit into a modified compensation value which sinusoidally changes with time; a circuit for generating a control command value by superimposing the modified compensation value generated by the waveform transforming circuit on the target current value determined by the target current determining circuit; and a motor driving circuit for driving the electric motor on the basis of the control command value.
With this arrangement, the compensation current value is generated according to the time-based differential value of the steering torque, and then subjected to the waveform transformation thereby to be converted into the modified compensation value which sinusoidally changes with time. Then, the modified compensation value obtained through the waveform transformation is superimposed on the target current value determined according to the steering torque for the generation of the control command value. Even if the steering torque changes slowly with time so that the compensation current value changes in a pulse form, the control command value does not change in a pulse form but gradually sinusoidally changes. Therefore, uncomfortable vibrations and noises of the electric motor or the steering mechanism can be prevented by controlling the driving of the electric motor on the basis of the control command value.
The present invention is particularly advantageous where the target current determining circuit, the compensation value determining circuit and the like are adapted to determine the target current value and the compensation current value in a digital data form on the basis of the steering torque in a digital data form.
The target current determining circuit, the compensation value determining circuit, the waveform transforming circuit and the like may perform their functions on a software basis by causing a microcomputer to execute predetermined programs.
The foregoing and other objects, features and effects of the present invention will become more apparent from the following description of the preferred embodiments with reference to the attached drawings.